Living Cationic Polymerizations

In some cationic polymerizations, when conditions are carefully controlled, quasi termination less or termination less systems can be achieved [130–136]. The “living” polymers, or “quasiliving,” form as chain transfer to monomer and all other forms of termination are greatly decreased or made reversible throughout reaction. The important aspect of living cationic polymerization is that the propagating centers are sufficiently low in reactivity. Transfer and termination reactions are suppressed. The propagation, however, is maintained. The molecular weights must increase in proportion to the cumulative amount of the monomer added. The lifetimes of the propagating species can be extended in such polymerizations by carrying out continuous slow additions of the monomer. Because chain-transferring reactions are not completely eliminated in all these systems, the term “quasi” is sometimes used. Living cationic polymerizations have been carried out with a number of monomers, such as isobutylene, styrene, p-methyl styrene, p-methoxystyrene, N-vinylcarbazole, and others [137, 138]. To achieve living conditions, it is necessary to match the propagating carbon cation with the counterion, the solvent polarity, and the reaction temperature. Some examples are offered in Table 4.2. By stabilizing the inherently unstable carbocationic growing species and preventing chain transfer and termination, living cationic polymerizations are achieved. This can be done by two methods:(1) use of suitable nucleophilic counterions, or(2)external additions of weak Lewis bases [139].The ion pairs are very tight in these reactions and may border on being covalent. The propagation can be illustrated as follows:

As shown above, the carbon–iodine bond is stretched, with or without the help of a Lewis acid. The Lewis acid assists in further stretching the carbon–iodine bond. Whether it is needed, depends upon the strength of that bond. This strength, in turn, varies with the ability of the solvent, the temperature, and the substituent R to stabilize the d+ center. Depending upon conditions, a Lewis acid can convert the counterion, like I to a more stable, less nucleophilic species. Lewis bases, like dioxane or ethyl acetate, may function by reacting directly with the propagating centers. Living cationic polymerizations were also carried out with heterogeneous catalysts. Thus, Aoshima and coworkers [139] reported a heterogeneous living cationic polymerization of isobutyl vinyl ether, using Fe₂O₃ in conjunction with the isobutyl vinyl ether-HCl adduct in toluene. This was done in the presence of an added base at 0C. Such bases are ethyl acetate and 1,4-dioxane. These bases are effective in homogeneous living cationic polymerization of vinyl ethers in the presence various metal halides. The living cationic polymerization of isobutyl vinyl ether produced polymers with very narrow molecular weight distribution of the product [139]. The number average molecular weight increased in direct proportion to the monomer conversion, and they reported that the molecular weight distributions were very narrow throughout the polymerization [Mw/Mn ¼ 1.1]. Stereoselectivity of the products was similar to polymers obtained by soluble catalysts. Controlled polymerization also occurred even at higher temperature (30C). What is particularly interesting is their report that they separated the catalyst from the mixture by centrifugation, and then used this catalyst to catalyze a second living polymerization under the same conditions, yielding a polymer with narrow molecular weight distribution. The ease of the catalyst separation permitted repeated reuse of the catalyst. Up to the fifth use, the catalyst maintained its activity to give well-defined polymers with very narrow MWD [139]. Heterogeneous conditions, due to poor solubility of heteropoly acid, in polymerization of isobutyl vinyl ether with H₃PW₁₂O₄₀ in CH₂Cl₂ were also studied. When bases like 1,4-dioxane or tetrahydrofuran were present the molecular weight distributions were very broad. By contrast, polymerizations in the presence of dimethyl sulfide at 30C yielded living polymerizations of the ether. Here too, the product had very narrow molecular weight distribution [139]. In summary, some typical features of living cationic polymerizations are: 1. The quasiliving or living/controlled carbocationic polymerizations are characterized by an ionization equilibrium between active and reversibly deactivated chains that are dormant. 2. The number average molecular weight of the polymers that form is proportional to the amount of monomer introduced into the reaction mixture. In most cases, the reactions are rapid, often to a point that it is impossible to stop them before all the monomer is consumed. 3. The concentration of the polymers formed is constant and independent of conversion. This concentration is often equal to the concentration of the initiator.

4. Addition of more monomer to a completed polymerization reaction results in further polymerization and a proportional increase in molecular weight. Storeyet al. [140] point out that in living polymerizations of monomers like isobutylene that are co-initiated by TiCl4 at temperatures as low as 80C, the livingness is limited not by chain transfer to monomer but rather by a unimolecular termination process. Unimolecular terminations often involve b-proton expulsions to produce polymers with terminal unsaturation. They claim, however, that this does not happen here. Rather the normal tert-chloride chain ends of polyisobutylene formed by this type of polymerization gradually become depleted. They propose, therefore, that an isomerization mechanism takes place instead in the presence of an active Lewis acid, under monomer starvation conditions. It can be illustrated as follows [140]:

Storey et al. [140] reported that the rate of depletion of tert-chloride end groups follows first-order kinetics with an apparent rate constant of 8 10 5 s 1. The ratio of rate constants for propagation and rearrangement, kp/kR was found by them to be 3 10 4 M1. Fedor et al. [141] also studied the living cationic polymerization of isobutylene using 2-chloro 4,4,4-trimethylpentane/TiCl4/2,6-di-tert-butyl-pyridine system in hexane/methyl chloride (60/40 and 40/60) solvent mixtures at various temperatures ranging from 25 to 80C temperatures. Their structural analysis of products obtained at 40C using 1H NMR spectroscopy shows the presence olefinic end groups, increasing in content with increased conversion. This led them to conclude that termination in polymerizations carried out at higher temperature does involve terminative chain transfer or b-proton elimination from the living chain ends. They found that the eliminated proton is trapped instantly by a proton trap, 2,6-di-tert-butylpyridine [141]. Aoshima and coworkers [142] investigated the living polymerization of isobutyl vinyl ether using pyrrole, metal halides and a weak Lewis base. In conjunction with oxophilic acids such as ZrC14, long-lived species were produced to yield polymers with narrow molecular weight distributions and number average molecular weight values based on the used amounts of pyrrole. Acid-trapping experiments using silyl ketene acetal, indicated that pyrrole worked not as an initiator but as a transfer agent. The polymerization started from moisture reacting with zirconium chloride, followed by the reactions between the propagating species and the 2- and 5-positions of pyrrole, accompanied by the generation of HCl. In addition to the propagation from the generated HCl, the produced pyrrole-bonded chain-end structures were also activated by the oxophilic chlorides to generate propagating carbocationic via the abstraction of the iso butoxy group. As a result, the number of growing chains increased. Such transfer reactions occur predominantly in the early stage of the polymerization stemming from the highly nucleophilic nature of pyrrole. Thus, the resulting polymers have expected molecular weight values and narrow molecular weight distribution [142].

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مواضيع ذات صلة

Pseudo-Cationic Polymerization

Steric Control in Cationic Polymerization

Propagation in Cationic Polymerization

Electro Initiation of Polymerization

Charge Transfer Complexes in Ionic Initiations

One Electron Transposition Initiation Reactions

Metal Alkyls in Initiations of Cationic Polymerizations

Lewis Acids in Cationic Initiations

Initiation by Protonic Acids

Cationic Polymerization

The Chemistry of Ionic Chain-Growth Polymerization

Polymer Preparation Techniques